The present invention is directed to an optical pump used with a remote amplifier. More particularly, the present invention is directed to an optical pump that produces light at a plurality of wavelengths.
Optical transmission systems have recently been developed that utilize remote amplifiers to amplify optical signals. FIG. 1 illustrates a section of an exemplary optical transmission system that includes remote amplifiers.
The optical transmission system 5 includes a pair of optical fibers 11, 12 on which optical signals travel in the direction indicated by the arrows. Multiple repeaters are placed along the fibers 11, 12. One such repeater 10 is shown in FIG. 1. The repeater 10 includes a high-powered optical pump 14. Coupled to the high-powered optical pump 14 is a remote pump fiber 15 that extends externally from the repeater 10. The remote pump fiber 15 includes branches 16, 18.
The transmission system 5 further includes multiple remote erbium doped fibers (EDFs) 22, 28 disposed along the fibers 11, 12. Each remote EDF 22, 28 is coupled to a section of the remote pump fiber 15 and to an optical isolator through a wavelength division multiplexer (not shown). For example, erbium doped fiber 22 is disposed on fiber 11 and is coupled to remote pump fiber branch 16 and optical isolator 24. Erbium doped fiber 28 is disposed on fiber 12 and is coupled to remote pump fiber branch 18 and optical isolator 26. The arrangement of remote pump fibers and erbium doped fibers amplifies the optical signals on the fibers 11, 12 in a known way.
In the optical transmission system 5, it is desirable for the optical signal supplied by the pump 14 to the remote pump fiber 15 to have as large amount of power as possible for multiple reasons. First, as the power is increased at the input of the fiber 15, power at the output of the fiber 15 (at remote pump fiber branches 16, 18) where it is coupled to the erbium doped fibers 22, 28 is increased, thus increasing the amount of amplification provided by the erbium doped fibers 22, 28.
Further, as the power is increased, the length of the remote pump fiber 15 can be increased and therefore the distance between the pump 14 and the fibers 22, 28 can be increased.
Finally, as the power is increased, more branches similar to remote pump fiber branches 16, 18 can be coupled to the remote pump fiber 15 and used to pump additional remote erbium doped fibers.
However, optical fibers are limited in how much power they can carry. This limitation is caused by the effects of Raman gain and Raman noise.
In FIG. 2, the curve 32 is typical of the spectral dependence of the Raman gain coefficient for a silica optical fiber pumped at 1480 nm (see Govind P. Agrawal, Nonlinear Fiber Optics, Second Edition, Academic Press, 1995, pg. 318). The resultant gain spectrum in a lossless fiber is related exponentially to the gain coefficient by: ##EQU1## where P is the optical pump power and L/A is the effective length divided by the effective cross-sectional area of the fiber and is the property of the particular optical fiber. The curve 32 is centered around 1583 nm and has a spectral peak of approximately C.sub.4.
The resultant Raman noise spectrum is similar to the Raman gain spectrum and is centered around 1583 nm, the Stokes-shifted wavelength. However, light at this wavelength is of no use in providing pump power to the remote erbium doped fibers 22, 28 because it is at the wrong wavelength. Therefore, the generated Raman noise is an undesirable byproduct of the remote pump 14 because it reduces the power of the useful light that is supplied to the remote erbium doped fibers 22, 28. Further, the amplitude of both the Raman noise and the Raman gain in the fiber 15 increases exponentially as the amplitude of the input pump 14 is increased.
In addition to being subject to Raman gain, the Raman noise is also reflected inside the fiber 15 through the process of Rayleigh reflection. These reflections cause the generated noise to be subject to more gain that it would otherwise. When the fiber's gain reaches a threshold, the fiber 15 will lase at the peak Stokes-shifted wavelength. This is a limiting condition where an overwhelming amount of pump power is converted to the Stokes-shifted wavelength. Furthermore, the gain spectrum available for the amplification of signals is limited by the extent of the Raman gain spectrum shown in FIG. 2.
Based on the foregoing, there is a need for an improved high-powered pump that enables additional power to be input into the remote pump fiber 15.